Direct methanol fuel cell

ABSTRACT

A direct methanol fuel cell has a proton conducting membrane (PCM), a catalyst in contact with the PCM, a gas diffusion layer in contact with the catalyst, and a conducting plate in contact with the gas diffusion membrane. The gas diffusion layer comprises a microporous membrane.

FIELD OF THE INVENTION

The invention is directed to a direct methanol fuel cell (DMFC).

BACKGROUND OF THE INVENTION

The direct methanol fuel cell (DMFC) catalytically oxidizes methanol togenerate electricity. The DMFC differs from PEM (proton exchangemembrane) or solid polymer fuel cells, which use hydrogen gas forgenerating electricity. One major advantage of the DMFC over the PEMfuel cell is its ability to use methanol, a relatively inexpensive andeasily handled material when compared to hydrogen gas. One majordisadvantage of the DMFC, when compared to the PEM fuel cell, ismethanol crossover. Methanol crossover occurs when methanol from theanode crosses to the cathode. This causes the loss of efficiency of thecell. Nevertheless, the DMFC appears to be a viable portable powersource for devices such as cellular or mobile telephones, and handheldor laptop computers. “Types of Fuel Cells,” Fuel Cells 2000,www.fuelcells.org; Thomas, et al, “Fuel Cells-Green Power,” Los AlamosNational Laboratory, LA-VR-99-3231.

The DMFC is an electrochemical device. The anodic catalyzed reaction is:CH₃OH+H₂O→CO₂+6H⁺+6e⁻The cathodic catalyzed reaction is:3/2O₂+6H⁺+6e⁻→3H₂OThe overall cell reaction is:CH₃OH+ 3/2O₂→CO₂+2H₂OThese cells operate at efficiencies of about 40% at temperatures of50-100° C., the efficiencies will increase at higher operatingtemperatures. Fuel Cells 2000, Ibid; Thomas, Ibid.

As with any chemical reaction, reactants, products, and unwantedproducts (by-products) become mixed as the reaction proceeds, andseparation of these materials is an engineering challenge. So, at theanode, methanol, water, and carbon dioxide will be mixed together. Onemust be careful that excess methanol not accumulate at the anode becauseit will crossover the proton conducting membrane (PCM) and decrease thecell's efficiency. Water is good for the PCM, which needs water tomaintain its proton conductivity, but if water accumulates, it canprevent methanol from reaching the catalyst, or it can be recycled backinto fuel mixture where it can dilute the fuel. Both can decrease theefficiency of the cell. Carbon dioxide (or CO_(x)s) must be removed toallow room for the fuel at the anode. Otherwise, cell efficiency cansuffer.

Likewise, at the cathode, oxygen typically from air, must reach thecathode and water must be removed. If oxygen cannot reach the cathode,efficiency drops because the cathode half cell reaction is impeded. Ifwater, which can be used to moisten the PCM, is allowed to accumulate,it will prevent oxygen from reaching the cathode.

One challenge related to the foregoing is managing the reactant/productissues without greatly increasing the size or weight of the DMFC. DMFCis targeted, in part, at a portable power source for cellular or mobiletelephones and handheld or laptop computers.

In WO 02/45196 A2, a DMFC is disclosed. Referring to FIG. 3, the DMFC 40has proton conducting membrane (PCM) 80 with CO₂ conducting elements 52.On the anode side 41, there is a conducting plate 23 that has a flowfield 25, a gas diffusion layer 44, and an anodic catalyst 42. On thecathode side 31, there is a conducting plate 33 with a flow field 35, agas diffusion layer 48, and a cathode catalyst 46. The catalyst, anodeor cathode, is applied to either a surface of the PCM 80 or to the gasdiffusion layers 44, 48. The respective flow fields are in communicationwith their respective gas diffusion layers and the combined action ofthese flow fields and diffusion layers is intended to ensure the evendistribution of reactants to the catalyst and the efficient removal ofunwanted products, by-products, and unreacted reactants for thereaction. The gas diffusion layers are made of carbon fiber paper and/orcarbon fiber cloth and may be “wet-proofed” with PTFE polymer. Note thatthe gas diffusion layer, catalyst, and PCM are in close contact topromote electrons or protons conductivity.

On the anode side, fuel (methanol, methanol/water in either liquid orvapor form) is introduced at one end of the flow field 25, and byproducts (water, CO₂, and un-reacted fuel) are removed at other end ofthe flow field 25. CO₂ produced at the anode is intended to cross thePCM 80 via CO₂ conductors 52. Water produced at the anode is not meantto remain in the gas diffusion layer 42 as is apparent from the use ofthe PTFE. On the cathode side, air (the source of O₂) is introduced atone end of flow field 35, and water, unreacted air, and CO₂ are removedat the other end of flow field 35. Water produced at the cathode is notintended to remain in the gas diffusion layer 48 as is apparent from theuse of the PTFE.

In U.S. Patent Application Publication 2002/619253 A1, another DMFC isdisclosed. This DMFC is similar to the foregoing DMFC, except the carbonpaper or carbon cloth gas diffusion layers are replaced with a porousmetal layer. See paragraphs [0022-0024].

Accordingly, there is a need to improve reactant, product, andby-product management at both the anode and cathode of DMFC while notsignificantly increasing the size of weight of the DMFC.

SUMMARY OF THE INVENTION

A direct methanol fuel cell has a proton conducting membrane (PCM), acatalyst in contact with the PCM, a gas diffusion layer in contact withthe catalyst, and a conducting plate in contact with the gas diffusionmembrane. The gas diffusion layer comprises a microporous membrane.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form that is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a schematic illustration of a direct methanol fuel cell (DMFC)made according to the present invention.

FIG. 2 is a schematic illustration of a plurality of DMFC's connected inseries.

FIG. 3 is a schematic illustration of a plurality of DMFC's connected inparallel.

DESCRIPTION OF THE INVENTION

Referring to the drawings, wherein like elements have like numerals,there is shown in FIG. 1 a direct methanol fuel cell system 10.

DMFC system 10 includes a DMFC 12, a fuel source 14, and an electricalcircuit 16. DMFC may include one or more DMFC. Fuel source 14 is astorage vessel that contains the fuel, methanol, or a mixture ofmethanol and water. Electrical circuit 16 includes a switch 18 and aload 20. Load 20 may be any device that requires electricity, such as acellular or mobile telephone, or a handheld or laptop computer, or thelike. Fuel is supplied to DMFC 12 via line 22 from source 14 and isreturned to source 14 via line 24 from DMFC 12. Air is supplied to DMFC12 via line 26 and vented from DMFC 12 via line 28.

DMFC 12 includes a membrane electrode assembly (MEA) 30 preferablysandwiched between a pair of collection plates 32, 34. Collection platesare electrically conductive and are coupled to electrical circuit 16.Collection plate 32 includes a fuel distribution channel 36. One end ofchannel 36 is in fluid communication with line 22 and the other end ofchannel 36 is in fluid communication with line 24. Collection plate 34includes an oxidant distribution channel 38. One end of channel 36 is influid communication with line 26 and the other end is in fluidcommunication with line 38. The geometry of channels 36 and 38 is suchthat fuel or oxidant is even distributed to the catalysts of the DMFC12.

MEA 30 includes a proton conducting membrane (PCM) 40 with an anodecatalyst 42 on one side thereof and a cathode catalyst 44 on the otherside thereof and all sandwiched between gas diffusion layers 46 and 48.PCM 40 is conventional, for example NAFION® (perfluorosulfonicsubstituted polytetrafluorethylene (PTFE)) from DuPont, Wilmington, Del.or the materials set forth in WO 02/45196A2, incorporated herein byreference which include NAFION®-TEFLON®-phosohotungstic acid (NPTA),NAFION®-zirconium hydrogen phosphate (NZHP), polyetheretherketone,polybenzimidazole, polyvinylidene fluoride (PVDF). Anode catalyst 42 maybe adhered to a face of PCM 40 or adhered to the fiber surfaces of acarbon fiber mat or cloth. Likewise, cathode catalyst 44 may be adheredto the other face of PCM 40 or adhered to fiber surfaces of a carbonfiber mat or cloth. The anode and cathode catalyst are conventional andthe methods of adhering same are also conventional.

The gas diffusion layers 46 and 48 may comprise a microporous membraneor a laminate of a microporous membrane and carbon fiber substrate. Themicroporous membrane may take on several different forms, the ultimateform being dependent upon the desire function of the membrane. Functionsof the membrane will be dependent upon whether it is located on the fueltank, or the anode and or the cathode. Functions for membranes at thefuel tank include: allowing the fuel to directionally flow to the anodeand preventing the flow back of other anode components, reactants andreaction products. Functions for membranes at the anode include:allowing the fuel directionally flow to the catalyst; preventingaccumulation of water at the catalyst; facilitating removal of gaseousreaction products form the electrode; maintaining adequate water levelby preventing the back flow of the anode liquid to the fuel tank;helping to prevent accumulation of MeOH at the catalyst thereby reducingthe chance for methanol crossover. Functions for membranes at thecathode include: allowing directional flow of the oxygen or air to thecathode catalyst for electrochemical reaction, and preventing the waterloss of the fuel cell system.

Membranes suitable to address these functions include microporous ornonporous membranes, skinned membranes, symmetric or asymmetricmembranes, single or multi-layered membranes, and combinations thereof.Such membranes are known, see for example, Kesting, R., SyntheticPolymeric Membranes, 2nd Edition, John Wiley & Sons, New York, N.Y.(1985), incorporated herein by reference. Such membranes are made ofthermoplastic materials, such as polyolefins (polyethylene,polypropylene, polybutylene, polymethyl penetene and the like),polyamides (nylons), polyesters (PET, PBT and the like). The membranesmay be made by the Celgard® process or by a TIPS (thermally inducedphase separation) process or a wet (solvent extraction) process.Additionally, the membranes may have functional coatings, for example,hydrophobic or hydrophobic coatings. Such coatings are conventional. Themembranes may also be combined with perm-selective gels or polymers thatpreferably pass one or more of the reactants, products, or by-products.Such perm-selective gels or polymers are conventional. Such aperm-selective material could coat one or more sides of the membrane orbe sandwiched between membranes.

As an example of the foregoing, one may use an asymmetric membrane(pores with decreasing diameters from one surface of the membranes tothe other) that is coated with a hydrophobic material on the surfacewith the narrow pores. This membrane is preferably made frompolymethylpentene (PMP). This membrane, which could be used at eitherthe anode or cathode, would be placed in the MEA with the coated facetoward the PCM. Thereby, water that is a reactant at the anode and aproduct at the cathode would be retained around the PCM are available tomoisten the PCM so that its proton conductivity is maintained.

FIGS. 2 and 3 illustrate further embodiments of the invention. In theseembodiments, a plurality of DMFC's are joined together to form a stack50. In FIG. 2, the DMFC's 12 are joined in series. In FIG. 3, the DMFC's12 are joined in parallel.

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.

1. A direct methanol fuel cell comprising a proton conductive membrane acatalyst juxtaposed to said proton conductive membrane a gas diffusionlayer juxtaposed to and covering said catalyst, said gas diffusion layercomprising a microporous membrane a conducting plate juxtaposed to andcovering said gas diffusion layer and having a flow field therein, saidflow field being in communication with said gas diffusion layer wherebya fuel comprising methanol being catalytically converted to electricityby said cell.
 2. The cell of claim 1 wherein said gas diffusion layerfurther comprises a hydrophobic coating next to said catalyst.
 3. Thecell of claim 1 wherein said gas diffusion layer being an asymmetricmembrane.
 4. The cell of claim 3 wherein said pores of said asymmetricmembrane having a decreasing diameter across a thickness of saidmembrane and the narrower diameters being next to said catalyst.
 5. Thecell of claim 1 wherein said gas diffusion layer further comprises amultilayered membrane, wherein one said layer being a thermoplasticmicroporous membrane and another said layer being a perm-selectivelayer.
 6. The cell of claim 5 wherein said perm-selective layer being agel perm-selective material.
 7. A direct methanol fuel cell comprising aproton conductive membrane having a first face and a second face ananodic catalyst juxtaposed to said first face a cathodic catalystjuxtaposed to said second face a first gas diffusion layer juxtaposed toand covering said anodic catalyst, said first gas diffusion layercomprising a thermoplastic microporous membrane a second gas diffusionlayer juxtaposed to and covering said cathodic catalyst, said second gasdiffusion layer comprising a thermoplastic microporous membrane a firstconducting plate juxtaposed to and covering the first gas diffusionlayer and having a flow field therein, said flow field being incommunication with said first gas diffusion layer a second conductingplate juxtaposed to and covering the second gas diffusion layer andhaving a flow field therein, said flow field being in communication withsaid second gas diffusion layer whereby a fuel selected from the groupof methanol, or methanol/water introduced into the flow field of firstconducting plate diffuses to said anodic catalyst where electrons may begenerated in a known manner and protons crossover said proton conductivemembrane to said cathodic catalyst where said protons may be combinedwith oxygen in a known manner and thereby generate electricity.